net/mlx5: SF, Correct vhca context size

Fix vhca context size as defined by device interface specification.

Fixes: f3196bb0 ("net/mlx5: Introduce vhca state event notifier")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>